Welcome to iMore! Double check that you have an alert selected in Settings > Sounds and that you have sound notifications set to ON in Settings > Notifications > Mail. Also keep in mind that you can set different alert sounds for individual email accounts under Settings > Notifications > Mail > [email account]

Welcome to iMore! Double check that you have an alert selected in Settings > Sounds and that you have sound notifications set to ON in Settings > Notifications > Mail. Also keep in mind that you can set different alert sounds for individual email accounts under Settings > Notifications > Mail > [email account]